The present invention relates to frequency converters in general, and more particularly to static frequency changers in which the grouping of electronic power switches requires the use of interphase reactors.
The invention will be described in the context of an unrestricted frequency changer. A typical unrestricted frequency changer (UFC) has been described in U.S. Pat. No. 3,170,107 of R. D. Jessee.
The unrestricted frequency changer can be defined as a converter in which a plurality of controlled bilateral switches are operated in a repetitive switching pattern at a chosen repetition rate to apply successive increments of alternating voltages from a polyphase source to an output circuit to provide an output frequency which is a function of the difference between the input frequency and the repetition rate of switching. The switches are turned ON at a selected time and are turned OFF at a chosen time the latter being varied to change the conduction duty cycle of each bilateral switch so as to vary the output voltage of the frequency changer. The said selected time and chosen time are both varied concurrently so that the conduction duty cycle be varied as a function of the output frequency.
The unrestricted frequency changer (UFC) is a most desirable power conversion system since it affords the possibility of generating any frequency output for any frequency of the input supply by merely varying the rate of switching within the converter. Another desirable feature resides in that the input phase angle of an UFC is always the opposite of the output phase angle. Therefore, if the load is inductive at the output, capacitive load will appear at the input, and vice versa.
It is common practice with polyphase systems to divide the switches into separate groups relative to a common output phase, in order to be able to lengthen the time of conduction for the same current capability of the switch, thereby to increase the output of power at the common output. When this is done an interphase reactor is generally required for combining the current paths in the groups of switches and for outputting the total load current with less distortion. It is generally known in frequency converters, as well as in DC rectifier circuits, to use interphase reactors for that purpose. Interphase reactors are particularly advantageous in cycloconverters since these often combine several converters of banks of thyristors.
Cycloconverters are used for their low weight as airborne power generating systems where six-, nine-, or twelve-phase input power can be directly generated by appropriate alternators which readily allow the combination of a number of basic power circuits to reduce the ripple voltages in the output and thus to decrease the size of the output filter. Interphase reactors are typically shown in U.S. Pat. No. 3,431,483 of D. L. Lafuze. In all prior art arrangements, since each output phase must be derived independently from the other output phases, the interphase reactors are generally mounted on separate magnetic cores, one for each output phase.
In a polyphase system of this sort, the magnetic core of the interphase reactor represents weight and cost. This is particularly disadvantageous when the frequency changer is to be airborne.
The main object of the present invention is to widen the field of application of static frequency changers.
Another object of the present invention is to provide a novel and improved unrestricted frequency changer;
Still another object of the present invention is to provide a polyphase converter system of reduced weight and cost.